The present invention relates to subterranean treatment operations, and more particularly, to methods of degrading filter cakes in subterranean formations.
Often, once drilling of a well bore in a subterranean formation has been initiated, a fluid referred to as a “well drill-in and servicing fluid” may be employed. As referred to herein, the term “well drill-in and servicing fluid” will be understood to mean a fluid placed in a subterranean formation, such as those from which production has been, is being, or may be cultivated. For example, an operator may begin drilling a subterranean borehole using a drilling fluid, cease drilling at a depth just above that of a productive formation, circulate a sufficient quantity of a well drill-in and servicing fluid through the bore hole to completely flush out the drilling fluid, then proceed to drill into the desired formation using the well drill-in and servicing fluid. Well drill-in and servicing fluids often may be utilized, inter alia, to minimize damage to the permeability of such formations.
In some embodiments, well drill-in and servicing fluids may include “fluid-loss-control fluids.” As referred to herein, the term “fluid-loss-control fluid” will be understood to mean a fluid designed to form a filter cake onto a screen or gravel pack, or in some cases, directly onto the formation. For example, a fluid-loss-control fluid may comprise a comparatively small volume of fluid designed to form a filter cake so as to plug off a “thief zone” (e.g., a formation, most commonly encountered during drilling operations, into which the drilling fluid may be lost). Generally, well drill-in and servicing fluids are designed to form a fast and efficient filter cake on the walls of a well bore within a producing formation to minimize leak-off and damage. The filter cake often comprises an inorganic portion (e.g., calcium carbonate), and an organic portion (e.g., starch and xanthan). The filter cake generally is removed before hydrocarbons from the formation are produced. Conventional methods of removal have involved contacting the filter cake with one or more subsequent fluids.
Other conventional methods of removing the filter cake include formulating the well drill-in and servicing fluid so as to include an acid-soluble particulate solid bridging agent. The resultant filter cake formed by such well drill-in and servicing fluid then is contacted with a strong acid to ultimately dissolve the bridging agent. This method is problematic, however, because the strong acid often corrodes metallic surfaces of completion equipment (e.g., sand control screens), thereby causing such equipment to prematurely fail. Further, the strong acid may damage the producing formation. Additionally, the strong acid may cause the bridging agent to dissolve prematurely, resulting in the loss of the strong acid into the formation, before it can completely cover the filter cake.
Another method of filter cake removal has involved the use of a water-soluble particulate solid bridging agent in the well drill-in and servicing fluid, which bridging agent subsequently is contacted with an aqueous salt solution that is undersaturated with respect to such bridging agent. This method is problematic, however, because such bridging agents may require a relatively long period of time to dissolve in the aqueous salt solution, due to, inter alia, the presence of various polymeric material in the well drill-in and servicing fluids. Such polymeric material may prevent the aqueous salt solution from contacting the water-soluble bridging agents.
Operators also have attempted to remove the filter cake by contacting it with a combination of an acid and an oxidizer. The acid may be used to degrade the inorganic portion of the filter cake, while the oxidizer may be employed to degrade the organic portion. However, this may be unnecessarily expensive, as it involves placement of additional components into the formation, at additional cost. For example, operators have attempted to remove the filter cake by flowing a solution comprising hydrogen peroxide into the well bore and permitting it to contact the filter cake. This may be problematic, however, as the transportation, storage, and handling of hydrogen peroxide may present safety concerns.
Another method involves formulating a drill-in fluid to include an acid-soluble particulate solid bridging agent and then contacting the resultant filter cake formed by such drill-in fluid with a delayed-release acid system to ultimately dissolve the bridging agent. A common type of delayed-release acid system may comprise esters that slowly hydrolyze to form acids that may ultimately degrade the acid-soluble portion of the filter cake. This method of removing the filter cake may be problematic, however, because the acid may be prevented from dissolving at least a portion of the acid-soluble bridging agents, due to, inter alia, the presence of polymeric material in the well drill-in and servicing fluids. It is believed that this polymeric material may prevent the acid from contacting the bridging agent by acting as a protective type of coating on the bridging agents.